1. Field of the Invention
The present invention is directed to a method of preparing pyrochlore structure compounds. More particularly, the present invention is directed to a method of preparing lead-rich and bismuth-rich ruthenate and iridate pyrochlores in an alkaline reaction medium.
2. Description of Relevant Art
A number of electrochemical devices have been developed for producing electrical energy by electrochemical reaction and, conversely, for consuming electrical energy to effect electrochemical reactions. Many devices rely upon a reaction involving oxygen (or air) as part of the mechanism to accomplish the desired result. For example, such devices may contain oxygen electrodes, which are oxygen reducing cathodes, in which oxygen is catalytically electroreduced. Alternatively, such devices may contain oxygen electrodes which catalyze the evolution of oxygen from water. In general, these electrodes are known in the art as oxygen electrodes. Thus, metal-oxygen batteries, metal-air batteries, fuel cells, electrolyzers, metal electrowinning devices, etc., are among the well-known electrochemical devices which may contain oxygen electrodes. Typically, such devices contain electrocatalyst materials at one or more of their electrodes. For example, precious metals such as platinum (on carbon support) and silver (on carbon and other supports) are frequently employed as electrocatalysts.
Various electrocatalytic alloys, compounds and compound mixtures have been developed to enable such electrochemical devices to achieve more desirable systems. For example, U.S. Pat. No. 3,536,533 (Kitamura) describes the use of an alloy of gold, silver, palladium and at least one element selected from the group consisting of platinum, rhodium and ruthenium as a fuel cell electrode electrocatalyst. U.S. Pat. No. 3,305,402 (Jones et al) describes the use of a combination of platinum and ruthenium oxides as an electrocatalyst. However, both Kitamura and Jones et al describe these catalysts as fuel cell anode (or fuel oxidation) catalysts. In addition, O'Grady et al, Technical Report No. 37, "Ruthenium Oxide Catalysts For The Oxygen Electrode", Contract No., N0014-67-A-0404-0006 (AD-779-899) Office of Naval Research, May 1974 (National Technical Information Service) describes the use of ruthenium oxide as an electrochemical catalyst for both the generation and the reduction of oxygen. Also, U.S. Pat. No. 3,405,010 (Kordesch et al) teaches that spinel type electrode catalysts produces better activation of the electrode and improved electrolyte repellency of the electrode by the inclusion of ruthenium.
Thus, the foregoing prior art describes a variety of electrodes, including those which utilize iridium and/or ruthenium-containing catalysts. However, none of the references teaches or renders obvious the bismuth-rich and lead-rich pyrochlore compounds described herein, much less the particular method of preparation claimed herein.
Heretofore, many pyrochlore compounds such as the pyrochlore compounds Pb.sub.2 Ru.sub.2 O.sub.7-y (lattice parameter of 10.253 .ANG.), Pb.sub.2 Ir.sub.2 O.sub.7-y (lattice parameter of 10.271 .ANG.), Bi.sub.2 Ir.sub.2 O.sub.7-y, Bi.sub.2 Rh.sub.2 O.sub.7-y, Pb.sub.2 Rh.sub.2 O.sub.7-y, Pb.sub.2 Pt.sub.2 O.sub.7-y, Cd.sub.2 Re.sub.2 O.sub.7-y (commonly referred to as lead ruthenate, lead iridate, bismuth iridate, bismuth rhodate, lead rhodate, lead platinate and cadmium rhenate, respectively,) and similar compounds have been known. For example, Longo, Raccah and Goodenough, Mat. Res. Bull., Vol. 4, pp. 191-202 (1969), described the compounds Pb.sub.2 Ru.sub.2 O.sub.7-y and Pb.sub.2 Ir.sub.2 O.sub.7-y and their preparation at temperatures in excess of 700.degree. C. Sleight, Mat. Res. Bull., Vol. 6, p. 775 (1971) has also described the compounds Pb.sub.2 Ru.sub.2 O.sub.7-y and Pb.sub.2 Ir.sub.2 O.sub. 7-y (including the pyrochlore compound Pb.sub.2 Ru.sub.2 O.sub.6.5 having a lattice parameter of 10.271 .ANG.) and their preparation at 700.degree. C. and 3000 atmospheres. U.S. Pat. No. 3,682,840 (Van Loan) describes the preparation of lead ruthenate at temperatures of 800.degree. C. and higher. None of these references teach the existance of the lead-rich or bismuth-rich compounds made by the present invention or that such compounds can be prepared as claimed herein.
U.S. Pat. Nos. 3,769,382 (Kuo et al) and 3,951,672 (Langley et al) discloses a variety of techniques for preparing lead ruthenate and lead iridate at temperatures of at least about 600.degree. C., and preferably at higher temperatures. However, each reference fails to recognize that the lead-rich pyrochlores used in the present invention are obtained at generally lower temperatures or that such pyrochlores have improved physical properties. Further, both references fail to teach the present method of preparing lead-rich and bismuth-rich pyrochlore compounds.
Bouchard and Gillson, Mat. Res. Bull., Vol. 6, pp. 669-680 (1971) describe the preparation and properties of Bi.sub.2 Ru.sub.2 O.sub.7 and Bi.sub.2 Ir.sub.2 O.sub.7, including the high conductivity and small Seebeck coefficients of each compound. However, there is no teaching that these compounds are useful electrocatalysts in electrochemical devices. Derwent's Basic Abstract Journal, Section E, Chemdoc, Week No. Y25, Abstract No. 320 (Aug. 17, 1977), Derwent Accession No. 44866Y/25 describes electrodes for electrolysis of alkaline and carbonate solutions which comprise nickelplated steel strips coated with high conductivity layers containing Cd.sub.2 Re.sub.2 O.sub.7, Pb.sub.2 Re.sub.2 O.sub.7-y or Ni.sub.2 Re.sub.2 O.sub.7. These compounds are prepared by impregnating perrhenic acid and a metal nitrate such as Cd nitrate onto a nickel strip and baking at 350.degree. C. However, these compounds are all rhenates rather than ruthenates or iridates and are not taught to be lead-rich or bismuth-rich compounds prepared by the method of the present invention. National Bureau of Standards, Washington, D.C., Institute for Mat. Research, Abstract of Rept. No. NNSIR-75-742 (1974) describes the use of mixed oxides as oxygen-reducing electrocatalysts in acid fuel cells, including the use of barium ruthenate. However, the materials suggested for such electrocatalysts are not the pyrochlore type structure compounds made according to the present invention.
The foregoing prior art dealing with the synthesis of electrically conductive pyrochlore structure oxides teaches synthesis temperatures of at least 600.degree. C. While elevated temperatures have been considered necessary to overcome diffusional limitations encountered in solid state reactions, such temperatures result in the formation of sintered products with low surface areas. This is disadvantageous for materials used in catalytic and electrocatalytic applications since the concentration of available catalytically active sites is limited.
To conserve energy and maximize surface area, it would be desirable to synthesize electrically conductive pyrochlore compounds at significantly lower temperatures, e.g. below 300.degree. C. However, the kinetics of solid state reactions are unfavorably sluggish. Solution syntheses offer one possible approach to achieving these very low temperature reactions. For example Trehoux, Abraham and Thomas, Journal of Solid State Chemistry, Vol. 21, pp. 203-209 (1977) and C.R. Acad. Sc. Paris, t. 281 pp. 379-380 (1975) describe the solution preparation of a pyrochlore compound of the formula K.sub.1.14 Bi.sup.III.sub.0.27 [Bi.sup.III.sub.0.27 Bi.sup.V.sub.4.9 ] [O.sub.4.9 OH.sub.1.1 ] OH.sub.0.8. The synthesis is effected by adding a bismuth nitrate solution to a solution of 17% potassium hydroxide containing an excess of potassium hypochlorite. The reaction is carried out in this medium for 2 hours in a reflux type of apparatus at a temperature slightly higher than 100.degree. C. The synthesis and resulting product are different in many respects from the synthesis and product claimed herein. The compound prepared in the cited reference is not an oxide but rather an oxyhydroxide which has a significant amount of protons incorporated into the bulk structure. Proton nuclear magnetic resonance experiments show that compounds prepared according to the present invention are oxides which do not have significant amounts of protons incorporated into the structure. The pyrochlore synthesized by Trehoux et al is not a ruthenium or iridium containing compound and, in fact, is believed not to be an electrically conductive pyrochlore. The potassium hydroxide solution used in the Trehous reference serves not only as a reaction medium, but also as a constituent in the reaction since potassium is incorporated into the A site of the pyrochlore. In contrast, the alkali solution employed in the present invention is solely a reaction medium with no measurable amount of alkali metal cations incorporated in the pyrochlore compound product.
Morgenstern-Badarau and Michel, Ann. Chim., Vol. 6, pp. 109 et seq. (especially at 109-113) (1971), and C. R. Acad. Sc. Paris, Vol. 271, Seire C pp. 1313-1316 (1970) report the solution preparation of pyrochlore compounds having the formula Pb.sub.2 Sn.sub.2 O.sub.6 .times.H.sub.2 O where 0&lt;.times.&lt;1. The preparation conditions are strictly defined as follows: equimolar quantities of lead and tin are reacted from solution in the presence of the complexing agent nitrilotriacetic acid (NITA) such that the concentration of [NITA]/[Pb.sup.2+ ]=2. The pH of the reaction medium is fixed at 11 and the reaction is carried out for several hours at 80.degree. C. The compound prepared by Morgenstern-Badarau et al is a hydrated oxide whereas materials prepared according to the present invention are oxides. In addition, the pyrochlore prepared by Morgenstern-Badarau et al, while containing lead, is not similar to the lead-rich pyrochlore prepared according to the present invention. Further, the pyrochlore prepared by Morgenstern-Badarau et al is not a ruthenium or iridium containing pyrochlore and is not believed to be electrically conductive. Also, Morgenstern-Badarau et al specifically state that their method of preparation forms a solid product containing Pb.sup.2+. In contrast, the solid product formed according to the present invention contains a mixture of Pb.sup.2+ and Pb.sup.4+. While the presence of a complexing agent is required in the synthesis described in the cited reference, such a complexing agent is not required according to the present invention. Furthermore, the pH range of the synthesis medium specified in the present invention clearly differs from the operable pH range of the cited reference. In fact, the Morgenstern-Badarau and Michel, Ann. Chim., Vol. 6, pp. 109-124 (l971) reference clearly states that no solid product compound can be obtained if conditions which are coincident with those specified for the present invention (pH&gt;13.5, temperature=80.degree. C., zero concentration of complexing agent) are employed.
More recently, U.S. Pat. Nos. 4,129,525; 4,163,706; 4,176,094; 4,192,780; 4,203,871 and 4,225,469 to Horowitz et al form pyrochlore compounds in which the pyrochlore oxide is precipitated from an alkaline solution and then separated therefrom by filtration. The filtrate is washed with water and dried to yield the pyrochlore solids. In U.S. Pat. Nos. 4,124,539 also to Horowitz et al, the precipitate is not recovered by filtration, but instead, the liquid of the pyrochlore/alkaline solution suspension is evaporated to dryness and the resulting oxide is washed in alkali or acetic acid. While the alkaline precipitation medium is the same as that disclosed in the other Horowitz et al patents, the particular method of separating the pyrochlore oxide precipitate from the precipitation medium claimed herein is markedly different.
Therefore, in summary, there exists a formidable body of prior art describing the existence of various pyrochlores, their potential uses (including uses as dielectric materials) and describing various metals and metal oxides as electrocatalyst materials. Notwithstanding this prior art, there is no suggestion or teaching that (a) the lead-rich or bismuth-rich pyrochlore compounds made according to the present invention exist, or that (b) the present invention may be used to make such compounds.